Observing how others lift light or heavy objects: time-dependent encoding of grip force in the primary motor cortex

Abstract

During movement observation, corticomotor excitability of the observer's primary motor cortex (M1) is modulated according to the force requirements of the observed action. Here, we explored the time course of observation-induced force encoding. Force-related changes in M1-excitability were assessed by delivering transcranial magnetic stimulations at distinct temporal phases of an observed reach-grasp-lift action. Temporal changes in force-related electromyographic activity were also assessed during active movement execution. In observation conditions in which a heavy object was lifted, M1-excitability was higher compared to conditions in which a light object was lifted. Both during observation and execution, differential force encoding tended to gradually increase from the grasping phase until the late lift phase. Surprisingly, however, during observation, force encoding was already present at the early reach phase: a time point at which no visual cues on the object's weight were available to the observer. As the observer was aware that the same weight condition was presented repeatedly, this finding may indicate that prior predictions concerning the upcoming weight condition are reflected by M1 excitability. Overall, findings may provide indications that the observer's motor system represents motor predictions as well as muscular requirements to infer the observed movement goal.

Corticospinal excitability during imagined and observed dynamic force production tasks: Effortfulness matters

“…TMS studies on movement observation have shown that the observed movement-related effort modulates CSE in a transitive reach-grasp-lifting task (Alaerts et al, 2010(Alaerts et al, , 2012. Within this framework, we found a significant difference between the observation of an effort-related movement and a VC condition.…”

Observing object lifting errors modulates cortico-spinal excitability and improves object lifting performance

“…Our findings argue for the inclusion of observed kinematic information from prior lifts (evidently a reliable source of possible weight information for the sensorimotor system in this simple model). In support of this idea, (Alaerts et al, 2011) have shown that, in a blocked design, visual information about object weight from previously observed lifts drives cortico-spinal excitability before visual information about object weight in an upcoming observed lift is available. These low-level modulations may even serve a behavioural function, given the growing experimental evidence for a link between cortico-spinal excitability and motor output (Bagce et al, 2013;Klein-Flügge et al, 2013;Orban de Xivry et al, 2013).…”

“…we hope to explicitly test this prediction in future research by measuring lifting behaviour and cortico-spinal excitability at various time points throughout montages of observed lifts, when only a fraction of the kinematic error information is available (cf. Alaerts et al, 2011).…”

Simulating and predicting others’ actions

“…Another pertinent experiment that highlights the role of the motor system for action prediction is reported by Alaerts, de Beukelaar, Swinnen, and Wenderoth (2011). Using transcranial magnetic stimulations, the authors explored how the excitability of the observer's primary motor cortex (M1) changes according to the forcerequirements of an observed action.…”